Vetusols and paleosols: natural versus man-induced environmental change in the Atlantic coastal region of Morocco

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Surface Terrae Rossae (Rhodoxeralfs), and a Terra Fusca (Petrocalcic Palexeralf) developed on mid-Quaternary calcarenites in the moister region of Rabat, show substantial pedogenic formation of clay minerals, particularly kaolinite, mainly in the fraction <0.2 μm . For the most part, the pedogenic kaolinites show poor crystallinity of the fireclay type. They are mainly formed from feldspars and perhaps also amphiboles and pyroxenes in the fractions >2 μm , and from smectites inherited from the calcarenite-residual loam in the coarse (2– 0.2 μm ) and fine clay fractions ( <0.2 μm ). In one Terra Rossa soil, formation was interrupted twice by deposition of eolian materials, and it is regarded as a polycyclic pedocomplex. Eolian input was much less important during development of the other surface Terrae Rossae. On younger calcarenites ( ⩾100 ka ) near the coast, three Rendzinas (Typic Calcixerolls) show only slight mineral weathering and pedogenic clay formation, in spite of a varied spectrum of primary minerals. In one example, kaolinite formation has just started. The Calcixerolls were probably recalcified by addition of eolian material as a result of anthropogenic thinning of the vegetation and/or due to the vicinity of the coast. These results suggest that, at least in this region, the younger Terrae Rossae of the coastal area were formed over much of the Brunhes epoch. In areas farther from the coast they have been developing for almost all of the Brunhes epoch. Because climatic fluctuations in the Quaternary were generally slight in this region, the direction of soil formation was constant. Therefore we regard the Rhodoxeralfs in the northern part of our study area as Vetusols. According to our expanded definition, Vetusols are surface soils which underwent almost constant processes of soil formation under an almost constant constellation of soil-forming factors, especially climate and vegetation, over a long period of time, including at least some part of the Pleistocene. Consequently the present patchy distribution of Terrae Rossae is hardly a result of climatically induced periods of “geomorphological activity and stability”. but the result of the removal of an earlier dense forest cover, followed by severe soil erosion. The human impact on the ecosystem is much more serious than any climatic fluctuations. In the southwestern part of the study area, between El Jadida and Agadir, the soil moisture regime is dry xeric to aridic and Terrae Rossae occur only in small areas, mostly in depressions. However, two selected Rhodoxeralfs in the vicinity of Tamanar show considerable pedogenic mineral weathering and clay mineral formation, though hardly any kaolinite. The strong weathering and clay illuviation must have been preceded by decalcification, implying a distinctly moister climate here in the past. However, micromorphological features of recent recalcification are common in these Rhodoxeralfs, suggesting that their main properties are not in equilibrium with the present climate and the soils should be regarded as nonburied paleosols or relict paleosols.